The present invention is by no means the first to deal with systems of this kind. Indeed, numerous such systems have previously been proposed. However, so far as the present inventors are informed, all such previously proposed systems had characteristics which detracted from their usefulness, or their desirability, in one way or another or were not adequate to meet the practical requirements of such systems.
For example, some prior systems proposals involved the use of auxiliary signals which are audible to the subscriber when using the telephone for conventional conversational purposes. This is not only disturbing to the subscriber, but also contravenes the technical restrictions imposed by some telephone companies, or governmental agencies.
Other such prior system proposals involved the use of auxiliary signals which were above the range of audible frequencies. However, this created electrical filtering requirements within the system itself, and possibly also within the conventional telephone network, which were difficult to implement, and also costly, to the point of impairing the economic acceptability of the system.
Still other system proposals, in an effort to overcome drawbacks such as noted above, simply sacrificed some desirable system functions, such as reliable surveillance of the equipment at the subscriber's premises, or the like.
One prior system with which the present inventors are quite familiar is described in U.S. application Ser. No. 327,486 filed on Dec. 4, 1981 and now U.S. Pat. No. 4,442,320. This patent is assigned to the same assignee as the present application.
In the system described in U.S. Pat. No. 4,442,320, a central control unit interrogates various instrumentalities located at a plurality of telephone subscriber premises and responses representative of the status of the instrumentalities are transmitted from the subscriber premises to the central control unit. Interrogation of the instrumentalities at a particular subscriber's premises is interrupted while the telephone of that subscriber is in use ("off-hook"). In order to maintain surveillance of these instrumentalities, a low tone signal, at a frequency below the audible range, is transmitted from the subscriber's premises to the central control unit while the telephone is in use and so long as all the monitored instrumentalities at the subscriber's premises are in given states (e.g. non-alarm). When a single instrumentality at a particular subscriber's premises changes state (e.g. alarm), the transmission of the low tone signal ceases and interrogation of all of the instrumentalities at these premises is resumed even though the telephone is in use and the interrogation signals are audible. A user of the telephone does not mind hearing the audible interrogation signals because they have been initiated by an apparent change of state of one of the monitored instrumentalities.
In the specific system described in U.S. Pat. No. 4,442,320, the absence of the low tone signal continues until the particular instrumentality which caused the interruption of the low tone signal returns to its initial state. As a result, interrogation of all the instrumentalities at the particular premises continues during an off-hook condition of the telephone even though all the necessary information about the particular instrumentality causing cessation of the low tone signal has been obtained.
Under certain circumstances, the continuous absence of the low tone signal and the resulting continuous interogation during an off-hook condition of the telephone can be undesirable. For example, not all changes in the state of an instrumentality signal emergency-type alarms and, therefore, the continuous transmission of interrogation signals in the audible range during an off-hook condition of the telephone is unnecessarily disturbing to the users of the telephone. Also, a system set to give interrogation priority to alarm conditions of any time will continue to give priority to a particular premises from which an alarm condition is indicated as represented by the continued absence of a low tone signal. In fact, once an alarm condition has been indicated and interrogation and response signals have been transmitted to identify the specific alarm condition, a further indication of the same alarm condition, as represented by a continued absence of a low tone signal, is unnecessary.